In order to maintain networks or communication systems properly, for example, telephone transmission facilities and the like, numerous measurements are made of network and system parameters. Important among these are the measurements of frequency response and envelope delay distortion. To this end, what is commonly called envelope delay is measured over the frequency range of the facility being evaluated. Envelope delay is defined as the slope of the phase versus frequency characteristic of the transmission facility. In an ideal communications system, envelope delay is constant over the frequency band. However, in practical systems there are deviations in the envelope delay over the frequency band. These deviations from an arbitrary reference are defined as the envelope delay distortion of the facility.
Envelope delay measurements have been attempted employing a test signal including a plurality of tones or tone pairs spaced at predetermined frequencies in the frequency band of interest in order to simultaneously obtain a measure of envelope delay over the entire frequency band for the facility under evaluation, as described in U.S. Pat. No. 3,573,611, issued Apr. 6, 1971.
Problems common to the known prior measurement arrangements are errors in and repetition of measurements of envelope delay distortion of facilities on which noise, frequency shift, nonlinearity or other impairments are present. Moreover, it is important to obtain accurate and reliable measurements in the presence of subtle changes in the amount of intermodulation distortion on the facility under evaluation.
Test arrangements which address the problems of accuracy, reproducibility and reliability of measurements of transmission parameters, e.g., of envelope delay distortion and/or frequency response of a network or facility are described in our copending application, Ser. No. 107,787, filed Dec. 28, 1979 now U.S. Pat. No. 4,301,536, issued Nov. 17, 1981. These test arrangements employ a set of test signals, each test signal having a plurality of tones and each tone having frequency, amplitude and unique phase component values determined and assigned thereto in accordance with prescribed criteria. An ensemble of measurements is made while transmitting the set of test signals over the network or facility under evaluation. In turn, the ensemble of measurements is used to obtain values for envelope delay distortion.
In one measurement procedure employing the set of test signals, measurements are first obtained to determine values of predetermined characteristics of the facility under evaluation, for example, frequency shift and third order intermodulation distortion. Then, depending on the values of frequency shift and third order intermodulation distortion being within at least first, second or third predetermined limits, at least first, second or third predetermined fixed test procedures, respectively, are employed to obtain the desired measurements of frequency response and/or envelope delay distortion. Specifically, if the frequency shift and third order intermodulation distortion products are within the first limits a first test procedure is employed utilizing a set of test signals including one test signal having the plurality of tones and randomly assigned phase component values wherein a first fixed set of measurements is obtained and time-averaged and a first predetermined fixed ensemble of time-averaged sets of measurements is employed to obtain the desired measurements. If the frequency shift and third order intermodulation distortion values are within second predetermined limits, a second test procedure is employed utilizing a set of test signals identical to the first test procedure wherein a second fixed set of measurements is obtained and time-averaged and a second predetermined fixed ensemble of time-averaged sets of measurements is employed to obtain the desired measurements. If the frequency shift and third order intermodulation distortion values are within third predetermined limits, a third test procedure is employed utilizing a set of signals including a plurality of test signals each including the plurality of tones. Similarly, phase values are assigned to the tones on a random basis. After each transmission of a test signal, the phase component values are reassigned to the tones in accordance with a prescribed criterion. In this example, the phase component values are reassigned to the tones on a one-to-one basis in counterclockwise, circular fashion until each tone has taken on each phase component value. During individual test signal transmissions, a predetermined fixed set of measurements are obtained and time-averaged. Thereafter, the fixed ensemble of time-averaged sets of measurements is employed to obtain the desired measurement of envelope delay distortion. Once the ensemble of time-averaged sets of measurements of the test signals has been made, each time-averaged set in the ensemble is Fourier transformed to the frequency domain and the resulting spectrum is utilized to obtain the desired envelope delay distortion or frequency response measurement in accordance with predetermined procedures.
Although providing satisfactory results for some measurements, the prior fixed test procedures are not optimized for the specific impairments, e.g., noise, intermodulation distortion and frequency shift, of the network or facility being evaluated and may inefficiently use the system resources.